Just ssh into the camera app, replace the camera sounds with silent ones. You can try just deleting or appending the file name with a _ or something so the app can't find it when it goes to reference it, but from memory it caused my phone (Japanese 3GS) to freeze.
edit: Just found and attached the files I modified if you wanted to do it that way. Leave the originals on the phone and just append something to the filename so that you can change them back to the original if you need to. NB: It's actually the phone's UI audio folder you ssh into, NOT the actual camera app folder.